One common approach to convert a liquid hydrocarbon fuel into hydrogen for use in a fuel cell or other processes is to use autothermal reforming of the hydrocarbon. In this process, the hydrocarbon fuel is reacted with oxygen or air and water vapor at high temperature. The oxygen is combusted with the fuel to supply the energy needed to reach the required temperature and to supply the heat required by the endothermic reforming reaction. The water is added to provide the high steam-to-carbon ratio required by the reforming process. As the molecular weight of the hydrocarbon increases, for example from methane to a liquid hydrocarbon such as gasoline or diesel fuel, the level of steam required to maintain good catalyst activity also increases. Steam reforming of a hydrocarbon produces a mixture of CO and H2 as well as byproducts of CO2 and un-reacted H2O. Usually the CO level is high, requiring conversion to H2 by the water gas shift reaction, equation 1, which typically reduces the CO to about 1% due to the equilibrium limitation at the temperatures employed.CO+H2O═CO2+H2  eq. 1
The remaining CO must then be removed by a selective oxidation reaction with added O2. The level of CO must be reduced to below 10 ppm for use of the product stream containing H2 in a proton exchange membrane (“PEM”) fuel cell.
Typical operating temperatures for an autothermal reformer are in the range of 600 to 800° C. With these high operating temperatures, the product gases from the reforming reaction can also exit the reforming section at a temperature of 600 to 800° C. Achieving and maintaining this high temperature represents a sizable portion of the input energy, and effective use of this energy is needed to maintain good fuel processor efficiency. The overall system requirements, including an autothermal reformer reactor, water gas shift reactor, selective CO oxidation reactor, and multiple heat exchangers, results in a system that is large, heavy and difficult to operate.
In general, it is difficult to catalytically reform liquid hydrocarbons to produce CO and H2. Typically, this requires a high steam to carbon ratio to prevent the formation of coke or carbon on the catalyst surface, which will deactivate the catalyst and slow or stop the reforming reaction. While high levels of steam can be introduced into the catalytic reactor, this is not always desirable since it increases cost and for portable systems requires transportation of the extra liquid. The ability to reform liquid fuels becomes even more difficult when the liquid fuel contains sulfur, since the sulfur further deactivates the catalyst and requires even more steam to ameliorate this effect.
There is a need for a more compact and efficient autothermal reforming system for production of hydrogen gas from a hydrocarbon fuel. There is a need in a number of applications for a light and compact system for converting liquid fuels into hydrogen for use in a PEM fuel cell and for other devices and processes requiring hydrogen gas.